1. Field of the Invention
This invention relates to thin film photovoltaic devices, and more particularly, to an absorber layer for a thin film photovoltaic device that has a double-graded band gap, and methods of forming the same.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert light into electrical energy, and have been developed as clean, renewable energy sources to meet growing energy needs worldwide. The relatively high manufacturing costs associated with conventional crystalline silicon solar cells, which use thick substrates of high-quality material, is driving the development of large area thin film photovoltaic (TFPV) devices. TFPV devices may be formed from thin (<10 micron) films of amorphous, nanocrystalline, micro-crystalline, or mono-crystalline materials, and when fabricated on low-cost substrates, provide an economical alternative to conventional crystalline silicon solar cells.
TFPV devices that employ copper-indium-gallium-selenide (CIGS) absorber layers are of special interest, since CIGS absorbers have demonstrated high lab-cell efficiency (>20%) and large-area module efficiency (>15%). This is because CIGS films have a high absorption coefficient (i.e., approximately 105/cm), bandgaps in the range of 1.0 eV (for copper-indium-selenide) to 1.65 eV (for copper-gallium-selenide), and are efficient absorbers across the entire visible spectrum. Furthermore, CIGS absorbers generally consist of earth-abundant materials, making CIGS-based TFPV devices scalable for high-volume manufacturing.
“Double grading” the bandgap of the CIGS absorber is a method known in the art to increase the efficiency of CIGS solar cells. In a CIGS absorber layer that has a double-graded bandgap profile, the bandgap of the CIGS layer increases toward the front surface and toward the back surface of the CIGS layer, with a bandgap minimum located in a center region of the CIGS layer. The increasing bandgap profile at the front surface of the CIGS layer, i.e., the surface that receives incident light, discourages the generation of charge carriers near this surface, thereby reducing unwanted charge carrier recombination before the charge carriers can reach the appropriate electrode. The increasing bandgap profile at the back surface of the CIGS layer creates a back surface field, which reduces recombination at the back surface and enhances carrier collection.
Co-evaporation is one technique known in the art for producing a double-graded bandgap in a CIGS absorber layer. The co-evaporation process can produce a gallium (Ga) rich region at the front and back surfaces of a CIGS absorber layer and a gallium-poor region in the center of the CIGS absorber layer. However, co-evaporation is a relatively complex process that is not as economical or as easily implemented as other deposition processes known in the art. In a 2-step process, Cu—In—Ga metal precursors are deposited first, followed by a second selenization process to form a CIGS absorber layer. The 2-step process is generally more suited to large-scale low-cost manufacturing compared to the co-evaporation process. However, because gallium has slower reaction kinetics with selenium (Se) than with indium (In), gallium tends to accumulate towards the back surface of the CIGS layer during the selenization process, thereby creating a single grading in the bandgap profile, i.e., the bandgap of the CIGS layer increases from the front surface to the back surface. Double grading of the bandgap profile is then typically achieved by the incorporation of sulfur (S) at the front surface of the CIGS layer. However, sulfur incorporation adds considerable complexity to the deposition process and produces a TFPV absorber material (copper-indium-gallium-sulfur) of lower quality compared to CIGS.
In light of the above, there is a need in the art for an economical method of creating a CIGS absorber layer having a double-graded band gap that does not use sulfur incorporation.